1. Field
Circuit structures interconnecting individual devices of a circuit.
2. Relevant Art
One direction in improving integrated circuit technology is to reduce the size of the components or devices on a chip, permitting an increased number of devices on the chip. The reduction in size of the devices on an integrated circuit chip requires reductions in the widths and thicknesses of the interconnections that connect the devices on the chip.
At present, the combination of the interconnect's reduced cross-sectional area with the electrical current requirements of the transistors result in large current densities within the interconnect. It is known that large current density can cause migration of some of the interconnect material (ref. FM D'Heurle and A Gangulee, Thin Solid Films 25, p. 531 (1975)).
Migration of the interconnect material has been generally accepted to be the result of electrons colliding with the atoms within the interconnect. The collisions occasionally cause atoms to dislodge and move in the direction of the electron flow via one of three routes: interstitially, along grain boundaries, or along the free surface. If the migration flow of atoms away from the interconnect is greater than a flow of source atoms to the interconnect, a void will form. Growth of the void will eventually result in an opening being formed in the interconnect. The ability of the interconnect material to resist this failure mode is referred to as the electromigration resistance. Electromigration resistance is a primary factor limiting interconnect materials longevity. One way to increase performance, reliability, and power consumption of integrated circuit interconnections is by improving the electromigration lifetime.
Where three grain boundaries meet, a triple point junction is formed. Such junctions are randomly dispersed throughout the interconnection and extend in a variety of directions that define potential inlet and outlet routes for displaced copper atoms during current flow. As electrical current flows through the interconnection, copper atoms are displaced by the electrons. These displaced copper atoms accumulate in the grain boundaries that are downstream of the current and travel along the grain boundaries in the general direction of the current. At grain boundary junctions that have fewer upstream inlets than downstream outlets, a void may develop at that grain boundary junction over time as copper atoms erode form the junction.
FIG. 1 schematically illustrates a copper interconnection and shows a number of junctions created by adjacent copper crystals. Interconnection 70 is formed, in this example, by copper crystal 72, copper crystal 74, copper crystal 76, copper crystal 78, and copper crystal 80. Grain boundary junction 82 is formed by the meeting of inlet grain boundary 84, outlet grain boundary 86, and outlet grain boundary 88, the designation of inlet and outlet being dictated by the indicated direction of the flow of electrons. With one upstream inlet and two downstream outlets, more copper atoms can be expected to leave junction 82 through two downstream outlets 86 and 88 then are supplied to junction 82 through one upstream inlet 84. With more copper atoms being removed from junction 82 within interconnection 70 than are being supplied to junction 82 from its upstream source, here inlet grain boundary 84, void 90 will eventually develop in interconnection 70 at junction 82.
Modern interconnections are made principally of a polycrystalline metal consisting of copper, aluminum, or an aluminum alloy. Electromigration resistance of these metals may not be sufficient in future generations of integrated circuits due to the increased current density.
Several techniques have been developed to improve the electromigration lifetime of an interconnection. These techniques include improved texture, interlayers to limit void size, and interconnections of multiple layers of material.
The introduction of refractory metals into integrated circuits has been hindered by the inability to deposit the alloys using electrodeposition from an aqueous solution. It is not currently possible to directly plate refractory metals from an aqueous solution.
The features of the described embodiments are specifically set forth in the appended claims. The embodiments are best understood by referring to the following description and accompanying drawings, in which similar parts are identified by like reference numerals.